Template:CheckboxWithTimestamp

 <input type="checkbox" onclick="setTimestamp(this)" />
 

<script> function setTimestamp(checkbox) {

 const box = checkbox.nextElementSibling;
 if (checkbox.checked) {
   const now = new Date();
   const timestamp = now.toLocaleString(); // local date + time
   box.textContent = timestamp;
 } else {
   box.textContent = "";
 }

} </script>